The present invention relates generally to a holographic viewing device and a computer-generated hologram for the same, and more particularly to holographic spectacles designed such that zero-order light that reaches directly the eye of a viewer without being diffracted through a hologram can be seen in place of light sources in a scene, offering no adverse influence on a pattern, and a computer-generated hologram for the same.
U.S. Pat. No. 5,546,198 has proposed holographic spectacles constructed as shown in the perspective view of FIG. 6(a). As shown, two transmission holograms 2 and 3 are fitted in the two-eye sections of a spectacle frame 1. When the spectacles are used to view a scene including such limited extent light sources 4, 5, 6 and 7 as shown in FIG. 6(b), the user would see it as if shown in FIG. 6(c) as an example. In other words, the user would see the pre-selected patterns “NOEL” 8, 9, 10 and 11 in place of the light sources 4, 5, 6 and 7 in the natural scene of FIG. 6(b). For the transmission holograms 2 and 3 having such characteristics, Fourier transform holograms (Fraunhofer holograms) of the aforesaid pattern “NOEL” designed as computer-generated holograms are used.
In a computer-aided Fourier transform hologram, a limited rectangular area including a pattern (for instance, “NOEL” as mentioned above) recorded in that hologram is divided into a matrix array of cells, so that information regarding a pattern portion corresponding to each cell site can be allocated to each cell, so that a pattern comprising a limited number of cells is projected onto a hologram area far away from it upon Fourier transform. As is the case with the pattern recording area, the hologram area, too, is divided into a matrix array of cells to record amplitude information and phase information at each cell site where the pattern to be recorded is subjected to Fourier transform.
Thus, the Fourier transform computer-generated hologram with pre-selected patterns recorded in it is fabricated in such a way that the diffraction efficiency reaches 100% at a given (design) wavelength. Even so, there is zero-order transmitted light that reaches directly the eye of the viewer without being diffracted through the hologram, because any phase condition is never satisfied at other wavelengths. In a practically fabricated computer-generated hologram, too, there is such zero-order light because its phase distribution, etc. deviate more or less from design values and so the diffraction efficiency does not necessarily reach 100%. This zero-order light is seen as a spot at the center of the reconstructed pattern viewed in place of light sources in a scene, posing an obstacle to viewing.